The Bull Shark

The bull shark, Carcharhinus leucas, also known as Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers. The bull shark is well known for its unpredictable, often aggressive behavior.

Unlike most sharks, bull sharks tolerate fresh water and can travel far up rivers. They have even been known to travel as far up as Indiana in the Ohio River and Illinois in the Mississippi River, although there have been few recorded attacks. As a result, they are probably responsible for the majority of near-shore shark attacks, including many attacks attributed to other species.[1] However, bull sharks are not true freshwater sharks (unlike the river sharks of the genus Glyphis).

Distribution and habitat

The bull shark lives all over the world in many different areas and travels long distances. It is common in coastal areas of warm oceans, in rivers and lakes, and occasionally salt and freshwater streams if they are deep enough. It is found to a depth of 150 metres (490 ft) but does not usually swim deeper than 30 metres (98 ft). In the Atlantic it is found from Massachusetts to southern Brazil, and from Morocco to Angola. In the Indian Ocean it is found from South Africa to Kenya, India, and Vietnam to Australia.

There are more than 500 bull sharks in the Brisbane River; one was reportedly seen swimming the flooded streets of Brisbane, Queensland, Australia during the Queensland floods of late 2010/early 2011. Several were sighted in one of the main streets of Goodna, Queensland, Australia shortly after the peak of the January 2011 floods. There are greater numbers still in the canals of the Gold Coast, also in Queensland, Australia. A large bull shark was caught in the canals of Scarborough, 2 hours north of the Gold Coast. In the Pacific Ocean, it can be found from Baja California to Ecuador. The shark has traveled 4,000 kilometres (2,500 mi) up the Amazon River to Iquitos in Peru.

It also lives in fresh water Lake Nicaragua, and in the Ganges and Brahmaputra rivers of West Bengal and Assam in eastern India and adjoining Bangladesh. It can live in water with a high salt content as in St. Lucia Estuary in South Africa. After Hurricane Katrina, many bull sharks were sighted in Lake Ponchartrain. Bull sharks have occasionally gone up the Mississippi River as far upstream as Alton, Illinois. They have also been found in the Potomac River in Maryland.

Freshwater tolerance

The bull shark is the best known of 43 species of elasmobranch in ten genera and four families to have been reported in fresh water. Other species that enter rivers include the stingrays (Dasyatidae, Potamotrygonidae and others) and sawfish (Pristidae). Some skates (Rajidae), smooth dogfishes (Triakidae), and sandbar sharks (Carcharhinus plumbeus) regularly enter estuaries. Elasmobranchs' ability to enter fresh water is limited because their blood is normally at least as salty (in terms of osmotic strength) as seawater, through the accumulation of urea and trimethylamine oxide, but bull sharks living in fresh water reduce the concentration of these solutes by up to 50%. As a result, bull sharks living in fresh water need to produce twenty times as much urine as those in salt water.

Initially, scientists thought the sharks in Lake Nicaragua belonged to an endemic species, the Lake Nicaragua shark (Carcharhinus nicaraguensis). In 1961, following specimens comparisons, taxonomists synonymized them. They can jump along the rapids of the San Juan River (which connects Lake Nicaragua and the Caribbean Sea), almost like salmon.[16] Bull sharks tagged inside the lake have later been caught in the open ocean (and vice versa), with some taking as little as 7–11 days to complete the journey.

Anatomy and appearance

Bull sharks are large and stout. Females are larger than males. The bull shark can grow up to 3.4 metres (11 ft) (though a 13-footer was recently caught in the South Africa River) and weigh 230 kilograms (510 lb). Bull sharks are wider than other requiem sharks of comparable length, and are grey on top and white below. The second dorsal fin is smaller than the first. Per the television program Animal Face-off on the Discovery Channel Bull sharks with their sharp serrated teeth have a bite force of 1250 lbs.

Reproduction

Bull sharks mate during late summer and early autumn, often in the brackish water of river mouths. After gestating for 12 months, a bull shark may give birth to 4–10 live young. They are viviparous. The young are about 70 cm (27.6 in) at birth and take 10 years to reach maturity.

Ecology

Bull sharks are apex predators, and rarely have to fear being attacked by other animals. Humans are their biggest threat. Larger sharks, such as the tiger shark and great white shark, may attack them. Saltwater crocodiles have been well documented as regularly preying on bull sharks in the rivers and estuaries of Northern Australia. It is possible that other large crocodilians, such as the Nile crocodile and the American crocodile (both of whom share virtually all of their range with the bull shark) exhibit similar predatory behavior.